1. Field of the Invention
The present invention relates to a pellicle and more particularly, to a pellicle frame, which has the frame body made of a material containing silver or coated with a layer of silver that absorbs sulfide released from other materials of the pellicle preventing formation of crystals on the mask pattern of the photomask after exposure.
2. Description of the Related Art
IC (Integrated Circuit) is one of the most important elements that construct the so-called “third wave revolution” or “information revolution”. Computer, mobile phone, Internet, and LCD are important inventions of this digital era that greatly influence the living of human beings. Because IC chip has a wide application, it is used in a variety of electronic consumer products including computer and mobile phone. Following fast development of semiconductor technology, electronic products are designed to meet the requirements of modern electronic features such as light, thin, short, small, high speed, high frequency, high performance, and high precision. Heavy market demand for electronic products having modern electronic features promotes development of semiconductor technology towards this market trend. In consequence, investment in semiconductor industry keeps increasing in recent years. Every manufacturer is trying hard to create new technology in order to take the leading place in the market so as to enjoy huge commercial profit from the market. In order to survive from severe market competition, it is important to reduce the cost and to improve the efficiency in this semiconductor field.
Due to Moore's law, the successability of technical improvement toward smaller line width CD value is determined subject to photolithographic techniques, and scanner is the key implement. Currently, 248 nm deep-UV is intensively used for 0.11 μm photolithography. However, due to wavelength's sake, it is not possible to have the downward going line be in the way like 90 nm˜65 nm. Further, the use of 248 nm deep-UV for 0.11 μm lithography requires the so-called PSM (phase shift mask) reticle, which is made of molybdenum (Mo) that is about 2˜3 times over the price of chromium (Cr). In order to obtain a relatively smaller line width, the wavelength of the exposure machine should be relatively shorter. Therefore, 248 nm deep-ultraviolet light is intensively used to substitute for 365 nm ultraviolet. Recently, there are manufacturers studying the use of 193 nm deep-ultraviolet photoresist and light source of ultra short wave (Argon fluoride excimer laser to generate 193 nm deep-ultraviolet light) to improve lithographic process to the stage of 0.13 μm˜65 nm.
However, when using a photomask, it is provided with a pellicle to protect against top contaminant and bottom contaminant. If a photomask is used without a pellicle, adhesion of contaminants greater than the line-width of the electronic circuits of the mask pattern will destruct the function of the original circuit design. A pellicle comprises a pellicle frame and pellicle films provided at the top side of the pellicle frame and respectively adhered to the top and bottom sides of the photomask by means of adhesive means. Further, the inner side of the pellicle frame is coated with a layer of adhesive means for adhering contaminant in the enclosed space within the pellicle films and the pellicle frame so that the photomask pattern is protected against contaminants. It is well know that ArF lithography process ammonium sulfate crystals or haze will be generated on photomask surface. The ammonium sulfate comes from reaction of ammonia and sulfur oxides by the acceleration of 193 nm light energy. The source of ammonia may come from the environmental or the pellicle. The source of sulfur oxides may come from the environmental and the mask surface. The amount of the crystals will be increased and the accumulated crystals will become not transparent following the increasing of UV exposure, and such crystals will block light from passing through the photomask, resulting in a distortion of photomask pattern. This is a severe problem of the conventional pellicles.
Further, current semiconductor manufacturers commonly use SMIF system provided by Hewlett-packard for storing and transporting wafers/masks, i.e., the so-called enclosed transfer container. SMIF system is designed to reduce particle flux in storage and transport of semiconductor products during a semiconductor manufacturing process. This objective is achievable by: keeping the air proximity to the wafer or mask from change relative to the wafer or mask during storage and transport so as to prevent passing of particles from the surroundings into the air proximity to the wafer or mask. SMIF system uses a small amount of particle-free air to provide a clean environment for the object where the movement and flowing direction of the air and pollutant are well controlled. This measure greatly reduces the cost for clean room. Under the employment of SMIF system, the clean room of the foundry is maintained at Class 100, and the fabrication equipments are controlled to be under Class 0.1. Every fabrication equipment has a gate for the access of wafers/photomask, and enclosed transfer containers are used to transfer wafers/photomask. This method greatly reduces clean room installation cost and maintenance cost.
Further, before using 193 nm deep-UV to run a lithographic process, as shown in FIG. 1, a photomask A and a pellicle B are stored in an enclosed storage container D. When in use, the photomask A and the pellicle B are taken out of the enclosed storage container D and put in a mini-environment, and then radiated with 193 nm deep-UV. At this time, harmful crystals C are formed on the surface of the photomask A and the pellicle B (see FIG. 3). These crystals C lower the transmittance of the photomask A and the pellicle B, thereby resulting in distortion of the circuit pattern on mask and low yielding rate. Sometimes, the whole lot of wafers becomes unusable. This problem is indeed serious. This problem is also seen in the old manufacturing process with 365 nm ultraviolet light. However, because the old manufacturing process employs a relatively longer wavelength that has a relatively lower energy to provide a relatively lower capacity, the transparency of crystals formed on photomask after radiation is still high enough, and the problem of crystal formation on photomask during running of the old manufacturing process is never so serious to obstruct the product. According to experimentation, the transmittance of crystals formed on wafers after radiation with 365 nm is 76.1%; the transmittance of crystals formed on photomask after radiation with 248 nm is 29.2%, which is approximately the limit; the transmittance of crystals formed on wafers after radiation with 193 nm is 13%, which is about the opaque status. If this problem is not settled, semiconductor manufacturing process will be limited to 0.11 μm, and the unit transistor capacity will not be doubled as within 18 months as expected subject to Moore's law.
According to Example I in FIG. 2, the photomask A and the pellicle B were kept in an enclosed plastic storage container D at 40° C. for 3 days, and then the photomask A and the pellicle B were taken out of the enclosed plastic storage container D and put in a mini-environment and radiated with 193 nm deep-UV, and crystals C were found on the surface of the photomask A and the pellicle B. According to Example II in FIG. 2, the photomask A and the pellicle B were put in an enclosed plastic storage container D at 40° C. for 3 days, and then the photomask A was taken out of the enclosed plastic storage container D and put in a mini-environment and radiated with 193 nm deep-UV, and crystals C were found on the surface of the photomask A. According to Example III in FIG. 2, the photomask A and the pellicle B were put in an enclosed stainless steel storage container D at 40° C. for 3 days, and then the photomask A and the pellicle B were taken out of the enclosed stainless steel storage container D and put in a mini-environment and radiated with 193 nm deep-UV, and no crystal C formation was seen on the surface of the photomask A and the pellicle B. This study shows crystal formation has a great concern with the storage container, and pellicle frame will affect distribution of crystals on the glass and mask pattern of the photomask.
A pellicle is to be used to protect the mask pattern of a photomask against contaminants, preventing damage to the electronic circuits. However, the material of the pellicle frame and the surroundings provide a gas containing organic or inorganic sulfide that causes growth of crystals on the mask pattern. Observation shows distribution of crystals around the pellicle frame. According to the aforesaid Example II, even distribution of crystals can be observed. Therefore, preventing formation of crystals on the mask pattern requires improvement of the material for pellicle
According to study, we wound the reasons of crystal formation as follows.
1. According to analysis, the chemical formula of the crystals formed on the photomask and the pellicle is (NH4)2SO4, mainly composed of (NH4)+ and (SO4)2−. During synthesis, there are important catalysts: (a) light source of short wavelength and high energy, (b) organic or inorganic gas, (c) environment humility.
2. Either the use of Krypton fluoride excimer laser to generate 248 nm deep-ultraviolet light or Argon fluoride excimer laser to generate 193 nm deep-ultraviolet light, the narrow pulse light has a high energy that is continuously supplied during photolithography, which causes crystal formation upon its radiation on the photomask. It shows that the shorter the wavelength is, the higher the energy and the lower the transmittance of crystal will be.
3. In order to obtain high transmittancy of the pellicle, polymer of thickness below 0.002 mm is used for the film of the pellicle, which provides little insulation effect to gasses of relatively smaller molecules, and allows passing of wet air (water molecule) from the outside clean room into the inside of the pellicle to provide element requisite for its chemical reaction, and therefore crystals will be formed on the surface of the photomask and the pellicle after radiation with 193 nm deep-UV.
4. The material of the pellicle itself releases harmful gas in the enclosed space around the mask pattern thereby causing formation of crystals on the photomask and the pellicle after radiation with 193 nm deep-UV.
5. Because the pellicle frame is made of aluminum alloy treated with a sulfuric acid anodizing process, a porous membrane layer of capillaries is formed on the surface of the pellicle frame, and a big amount of sulphate ion (SO4)2− will be left on the surface of the pellicle frame, thereby increasing the concentration of sulfur molecules in the enclosed space between the chrome surface and the film upon evaporation of sulphate ion (SO4)2−, and further causing crystal formation on the photomask and the pellicle after radiation of the photomask and the pellicle with 193 nm deep-UV.
FIG. 4 is a schematic drawing, showing the use of a pellicle according to the prior art. As illustrated, light from a stepper passes through the photomask A and the pellicle B, causing the lens system F to map the image of the mask pattern E onto the image plane (photoresistant) H of the wafer G. In case the particle size of the crystals C or particles is smaller than the maximum tolerable size of the focal depth, the crystals or particles mean no harm. However, the crystals will grow with time and accumulation of exposure. If the growth of the crystals surpasses the maximum tolerable size of the focal depth, the crystals will affect the exposure.